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Nannochloropsis gaditana is recognized as a source of valuable pigments and polyunsaturated fatty acids (PUFAs), in particular eicosapentaenoic acid (EPA, 20:5ω3). The duration of the light period is an important environmental factor that affects the production yield in photosynthetic microorganisms. This factor was found to determine the growth and biochemical composition of this microalga strain. Cell division occurs during the first hours of the light period. Whereas carbohydrates are accumulated in the light period and consumed at night, the cellular concentration of chlorophylls and proteins does not vary significantly. The storage lipids are consumed in the lag phase (after dilution of the culture) and they increase until the final growth period. The structural lipids reach a maximum at the end of the daily exponential phase, when the cells also reach a maximum density, in a semi-continuous culture with a daily replacement of fresh medium.

Carotenoids synthesis takes place in the chloroplast but is catalysed by enzymes encoded by nuclear genes. These enzymes are thus synthesised in the cytoplasm as precursor polypeptides with an amino-terminal extension, the transit peptide (tp), and targeted to their final location in the chloroplast. β-carotene oxygenase (BKT) mediates the addition of keto groups to the position C4 of β-ione-rings of carotenoids. We studied the effect of the transit peptide sequences of Rubisco small subunit (RbcS2), Ferredoxine (Fd) or Phytoene desaturase (Pds) from Chlamydomonas corresponding proteins and the absence of tp on the functionality of exogenous bkt1 in transgenic Chlamydomonas reinhardtii. RbcS, Fd and Pds are targeted to the chloroplast stroma, intertylakoid lumen and tylakoid membrane respectively. Our results indicate that the presence of a chloroplastic transit peptide is essential for the efficient production of ketocarotenoids in transgenic strains of the microalga Chlamydomonas reinhardtii.